Passageways or conduits through a solid structure can be provided by depositing a base material for the structure onto a removable support having a profile shaped to provide the desired length and cross section of the passageway within the structure. Briefly, the base material is deposited onto the support so that when the support is removed, a passageway will remain. This concept can be used to form conduits or tubes of various sizes, nozzles and the like. Depending on the composition of the base material and the technique used to deposit the base material, the support should be resistant to high temperatures, caustic conditions, large shifts in temperature, and other conditions characteristic of the particular method used to deposit the base material.
One article that can be produced by the technique similar to the one briefly described above is a nozzle that accelerates and directs exhaust gases from a combustion chamber. In the past, nozzles of this type, particularly small nozzles having passageways with diameters on the order of 1.0 centimeter or less, have been made by depositing a base material on a mandrel made from a metal, for example, molybdenum. Metal mandrels normally require extensive and expensive machining to provide the desired shape within close tolerances. In addition, the metals used in the past had to be tolerant to high temperatures and were expensive. Because the mandrels are often destroyed to remove them from the deposited article, it is not economical to make them from expensive starting materials using costly manufacturing techniques.
Prior techniques regularly failed when making small ceramic nozzles or other conduits because of cracking related to differential thermal expansions between the mandrel and the deposited ceramic when the mandrel and deposited ceramic were cooled following deposition.